There is a reason for description being plaintext. We simply cannot anticipate every kind of HTML you may write (or worse, copy-paste) into the HTMLEditor. If you want to replace the Multiline Textbox with HTMLEditor, you should definitely take additional steps to sanitize the input from that field.
There are instances where theoretically valid HTML can screw things up a bit and there are definitely a number of places where invalid HTML will screw things up in possibly funny but very annoying ways.